2990 Chapter 24 



RACKING STRENGTH 



One measure of adequacy of sheathing panels is the deflection and strength of 

 walls sheathed with the panels, when the walls are subjected to in-plane shear, 

 i.e., to racking loads. Price and Gromala (1980) compared racking behavior of 

 flakeboards of high and low density and two thicknesses fabricated from 20 

 percent each of white oak, hickory, southern red oak, sweetgum, and southern 

 pine (see table 24-12 for fabrication details), with 1/2-inch yellow-poplar flake- 

 board, southern pine flakeboard (both with random orientation of flakes), and 

 with southern pine 3-ply, 1/2-inch-thick, CDX plywood. The racking loads were 

 applied to full-size racking panels (8 by 8 feet) according to ASTM E 72 

 (American Society for Testing and Materials 1968a) and also to small panels 

 measuring 2 by 2 feet. 



Price and Gromala found that when subjected to a 1 ,600-pound racking load, 

 8- by 8-foot panels sheathed with flakeboards containing a mixture of hardwood 

 and pine flakes were slightly stiffer than southern pine plywood (0.10-in. vs. 

 0.12-in. deflection). Yet, plywood sheathed panels provided slightly higher 

 strengths for the full-size racking test (6,000 vs. 5,500 pounds). The highest 

 average racking strength, 6,200 pounds, was obtained with the 1/2-inch yellow- 

 poplar-sheathed panels. 



After a 24-hour water soak, small-panel racking resistances and lateral nail 

 strength decreased. The racking strength decrease ranged from 4 percent (5/8-in. 

 oriented mixed high density) to 18 percent (1/2-in. oriented mixed low density). 

 All panel types had racking strength reductions within limits allowed under 

 current standards. The nail-strength decrease ranged from 9 percent (5/8-in. 

 random mixed high-density flakeboard) to 27 percent (1/2-in. southern pine 

 plywood). 



24-10 LINEAR EXPANSION 



Construction panels, particularly those used for roof sheathing and floor 

 decking, usually are rain wetted at times during the construction period before 

 roofing and walls are made weather tight. Also, all sheathing and decking 

 material is subject to fluctuations in relative humidity during construction and 

 during building life. Moisture pickup resulting from rain wetting or humidity 

 changes causes flakeboard to expand (fig. 24-42). The sorption isotherm for the 

 flakeboard described in figure 24-42 is depicted in figure 8-18. To guard against 

 buckling, application specifications for sheathing and decking panels call for 

 them to have end and edge spacing of 1/8 inch. This space is 0.26 percent of the 

 4-foot width and 0.13 percent of the 8-foot length of the usual 4- by 8-foot panel. 



Kasper and Carroll (1979) found that phenolic-bonded structural particle- 

 boards exposed as floor decking during 6 weeks of winter weather in Vancouver, 

 British Columbia — or under conditions simulating that weather — underwent 

 less than half (38-percent) of the linear expansion of unrestrained board speci- 

 mens subjected to an ovendry-vacuum-pressure-soak cycle. This diminution of 



